The present invention relates generally to the field of polymer synthesis. In particular, the present invention relates to the field of solution polymer synthesis.
Polymers have been prepared by a variety of means such as solution polymerization and emulsion polymerization. Emulsion polymerization is advantageous in that polymer particles having small particle sizes and particle size polydispersities approaching 1 can be prepared. Thus, emulsion particles having a controllable uniform particle size can be prepared. However, emulsion polymers contain surfactants, typically ionic surfactants. For many polymer applications, such as paints, ionic surfactants used during emulsion polymerization pose no problems. However, for other applications, such as those in the electronics industry, such ionic surfactants are problematic.
One application of polymers in the electronics industry is in the formation of porous interlayer dielectric materials used in the manufacture of integrated circuits. As electronic devices become smaller, there is a continuing desire in the electronics industry to increase the circuit density in electronic components, e.g., integrated circuits, circuit boards, multichip modules, chip test devices, and the like without degrading electrical performance, e.g., crosstalk or capacitive coupling, and also to increase the speed of signal propagation in these components. One method of accomplishing these goals is to reduce the dielectric constant of the interlayer, or intermetal, insulating material used in the components. A method for reducing the dielectric constant of such interlayer, or intermetal, insulating material is to incorporate within the insulating film very small, uniformly dispersed pores or voids. Preferred are pores or voids having a diameter of less than or equal to 100 nm.
One known process of making a porous dielectric involves dispersing thermally removable solid particles, i.e. porogens, in a B-staged dielectric precursor, polymerizing the dielectric precursor without substantially removing the particles, followed by heating the dielectric material to substantially remove the particles and thereby leaving voids or free spaces in the dielectric material. Such voids reduce the dielectric constant of the dielectric material. See, for example, U.S. Pat. No. 5,895,263 (Carter et al.).
While other methods of preparing porous dielectric materials are known, they suffer from broad distributions of pore sizes, too large pore size, such as greater than 20 microns, or technologies that are too expensive for commercial use, such as liquid extractions under supercritical conditions.
Solution polymer particles are known. For example, U.S. Pat. No. 5,863,996 (Graham) discloses a solution polymerization process including the steps of (i) polymerizing one or more monomers, the or at least one of which is a cross-linking agent, in a solvent which (a) has a solubility parameter from 2.5 cal1/2 ml−3/2 below to 1.0 cal1/2 ml−3/2 above the solubility parameter of the bulk polymeric material and (b) is of the same or adjacent hydrogen bonding group as the bulk polymeric material; (ii) monitoring the polymerization until polymeric material as herein defined is obtained; and (iii) terminating the polymerization before gelation is observed. This patent is directed to the formation of cross-linked, sol-forming particles without gel formation and discloses only particle sizes of no greater than 2 microns. There is no teaching in the '996 patent of how to control particle size, which is important in many applications, nor how to obtain cross-linked polymer particles having a particular particle size. Particularly, nothing in the '996 patent suggests how to prepare polymer particles having a mean particle size ≦20 nanometers.
There is thus a need for polymeric materials suitable for use as porogens to form porous materials, particularly porous dielectric materials, wherein the polymeric particles are substantially free of ionic surfactants and have a mean particle size of ≦20 nanometers.